1. Field of the Invention
The present invention pertains to a method of amplification based on spatio-temporal frequency drift for a pulse laser comprising a so-called CPA (Chirped Pulse Amplification) frequency-drift amplifying chain.
2. Description of Related Art
The production of pulse lasers, of titanium-doped sapphire type, with very large peak power, makes it necessary to control very wide spectra so as to decrease the durations of the pulses at the output of the amplifying chain.
In order to extract the largest part of the energy stored in the amplifying media, the latter are often used in a near-saturation regime. This saturation unfortunately causes in the frequency-drift chains a spectral shift which limits the total band.
A conventional solution for avoiding spectral constriction is to use a pre-compensation, at the start of the chain (before the regenerative or multi-pass amplifier). This filtering-based solution has the drawback of limiting the extraction efficiency of the amplifiers and is all the less effective the larger the number of passes through the amplifiers.
In detail, CPA chains implement frequency-drift technology which is based on the use of wide-spectrum pulses, the stretching of pulses, the amplification and re-compression of these stretched pulses. Typically, in CPA chains based on oscillators comprising Ti:Sa crystals which have a spectrum with a width of 5 to 100 nm, for compressed pulse durations of 150 to 10 fs.
The ability of an amplification chain to maintain a correct spectrum directly influences the ability of the laser to work with short pulses. The spectral constriction induced by the amplifiers is therefore a key factor for obtaining short-duration performance. Likewise, a large deformation of the spectrum, for example asymmetric, disturbs the temporal form and impairs the operation of the laser.
The amplifiers used are of the type with n passes of the beam through the amplifying medium. When n is small (less than 10) the geometric multi-pass configuration is generally used. The pump laser dispatches a pulse into the crystal and the beam to be amplified is thereafter dispatched and performs n passes so as to optimize the extraction in terms of energy.
FIG. 1 diagrammatically depicts a multi-pass amplifier such as this, which essentially comprises a crystal 1 (for example Ti:Sa) receiving, from an input mirror ME, input pulses at an angle differing from the normal to its incidence surface, and several reflecting mirrors M1 to M7 disposed on either side of the crystal 1 so as to cause the beam to pass through the crystal at various angles of incidence, the last mirror M7 reflecting this beam to the output via an output mirror MS.
When a large amplification factor is sought, it is necessary to increase the number of passes and the configuration of FIG. 1 is no longer applicable. The configuration generally used is then the regenerative amplifier, an exemplary embodiment of which is shown diagrammatically in FIG. 2. This type of amplifier makes it possible to readily achieve some thirty or so passes.
The system represented in FIG. 2 comprises a crystal 2 disposed, with a Pockels cell 3, in an optical cavity closed by two mirrors 4, 5 and pumped by a pump 6. A polarizer 7, disposed in the cavity, makes it possible to tap off a part of the intra-cavity beam, the tapped-off beam passing through a half-wave plate 8, a reflecting mirror 9 and a Faraday rotator 10 at the output of which a semi-transparent mirror 11 reflects it back towards the use (beam Eout). Moreover, the polarizer 7 makes it possible to inject an external beam Ein into this cavity.
In both cases (FIGS. 1 and 2), the gain of the amplifier may be written:
      E    OUT    =            J      SAT        ·    S    ·          ln      ⁡              (                                                            J                STO                                            J                SAT                                      ⁢                          (                                                ⅇ                                                            E                                              i                        ⁢                                                                                                  ⁢                        n                                                                                    SJ                      SAT                                                                      -                1                            )                                +          1                )            
JSTO being the stored fluence available for the gain in the medium (the crystal) and JSAT the saturation fluence of this medium. This is the classical equation from the theory of Frantz and Nodvick.
The table below contains a few examples of values of JSAT for various laser materials:
MaterialsJsat in J/cm2Spectral rangeDyes~0.001J/cm2VisibleExcimers~0.001J/cm2UVNd: YAG0.5J/cm21064 nm Ti: Al2O31.1J/cm2800 nmNd: Glass5J/cm21054 nm Alexandrite22J/cm2750 nmCr: LiSAF5J/cm2830 nm
In the small-signal regime, with JIN<<JSAT, the gain relation can be approximated with:
  G  =                    E        OUT                    E                  I          ⁢                                          ⁢          N                      =          ⅇ              (                              J            STO                                J            SAT                          )            
The amplified pulse being stretched (dispersed), usually positively, a problem has been highlighted by the Applicant. Specifically, chains based on short pulses use a wide-spectrum oscillator and these short pulses are stretched temporally and are thereafter amplified and re-compressed at the output. Such a chain is schematically represented in FIG. 3, this chain essentially comprising an oscillator 12, a stretcher 13, one or more amplification stages 14 and a compression device 15. An exemplary spectrum of a Ti:Sa oscillator signal has been represented in FIG. 4. In this FIG. 4, the spectral phase has been represented as a continuous line.
When the pulse penetrates the amplifier, the initial spectral components see a gain g1 and are amplified. The following components being in the amplifier therefore see a gain g2 which has decreased because the start of the pulse has “consumed” stored energy. The temporal form of the gain has a form of the type of that represented in FIG. 5.
There is an initial gain for the first temporal part of the form:
  gi  =            j      STO              J      SAT      
and a final gain, which takes account of the extracted energy, of the form:
  gf  =                    j                  STI          -                    ⁢              J        ex                    j      SAT      
Jex being the amplifier extracted fluence.
The apparent gain is therefore higher for the temporal start of the signal than for the end, thereby inducing a spectral deformation of the amplified signal, as represented in FIG. 6. The curve of FIG. 6 shows the effect of modifying the gain of a laser crystal due to the temporal stretching of the pulses to be amplified. This curve gives the value of the weighted gain (relative gain, as for all the other gain curves) as a function of the wavelength of the amplified signal.
FIG. 7 shows two curves of the shift of the gain due to temporal stretching as a function of wavelength, respectively for one pass and for four passes through the crystal.
In addition to the spectrum shift, a constriction due to the width of the gain band is also observed.
The combination of these two effects therefore greatly limits the performance of the frequency-drift chains, since it limits the re-compression of the incident pulses with a view to obtaining at the output pulses of very short durations, for example of a duration of a few fs.
To compensate for these effects, it is possible to undertake a pre-distortion of the input signal by active or passive filtering at the price of a decrease in the efficiency of the laser (drop in gain). Moreover, the filters used have low efficiencies (<50%) because they act (cut off) spectrally at the energy maximum.
A second solution would consist in having the amplifiers work far from saturation, but in this case the energy that can be extracted from the amplifier is greatly decreased. Moreover, the stability of the pulse at output then depends greatly on the stability of the input pulse.